Double-containment piping systems utilize an inner fluid carrier pipe disposed within an outer containment pipe. For application such as conveying toxic or dangerous chemicals, double-containment systems using thermoplastic pipes and fittings provide advantages over metal systems in their ability to withstand prolonged exposure to corrosive chemicals and in their lower cost and lighter weight. The outer pipe in these double-containment systems serves to prevent leaks and spills from the inner pipe from being released to the environment; and by providing detectors in the space between pipes, leaks in the inner pipe may be detected and repaired before the chemical escapes.
One of the problems presented in the design of double-containment thermoplastic piping systems is to provide compensation for differential thermal expansion that takes place when the inner and outer pipes are exposed to widely varying temperatures. This may occur where the inner pipe is carrying a heated liquid, and the outer pipe experiences a lower temperature as by being buried underground, or where the outer pipe undergoes wide temperature variations due to changes in ambient temperature in above-ground locations. The extent of difficulty encountered in compensating for thermal expansion varies with the composition of plastic pipes, with polyethylene presenting a high degree of difficulty owing to its high coefficient of thermal expansion and its characteristics of expanding up to 10 inches per 100 feet upon its being subjected to temperature changes such as 70.degree. F., which may be encountered in some applications. Another problem arising in underground systems feeding into a manhole is in obtaining a water-tight seal between the outer pipe and terminal connections to manholes which are normally made of concrete. Leakage has occurred at this point in prior systems owing to differences in expansion and contraction, a lack of chemical bonding between polyethylene and concrete and the smooth surface of the pipe, which does not resist movement.
Various measures for compensating for differential thermal expansion in thermoplastic double-containment piping systems are disclosed in the prior art. U.S. Pat. No. 4,786,088, issued Nov. 22, 1988, to Ziu, discloses an assembly in which restraint couplings rigidly connecting the carrier pipe and containment pipe to one another are placed in end-to-end relation with sections of the pipe. The assembly also discloses the use of a plurality of spaced-apart clips along the length of the pipe, the clips engaging the carrier pipe around a part of its circumference and having legs that engage the containment pipe. This patent is concerned with thermal expansion that occurs over long lengths of carrier pipe and with prevention of buckling of the carrier pipe against the containment pipe that would result from such expansion. Thermal expansion of the carrier pipe is said to be controlled by periodic positive connections to the containment pipe at the restraint couplings, and concentric alignment of the carrier pipe in the containment pipe is maintained by the support clips. Various other measures for controlling thermal expansion effects are disclosed, including thickening of fittings, use of expansion loops, and especially designed double-walled elbow and T-fittings. Such double-containment fittings have proven difficult and expensive to manufacture and install, particularly for larger diameter pipes. The patent also discloses that the assembly may be anchored to an external location, but no disclosure is given as to how this would be accomplished. In regard to the types of thermoplastic material employed in the assembly, the patent states that a carrier pipe and containment pipe may be formed from a polypropylene copolymer or from polyvinylidene fluoride, and no specific mention is made of polyethylene. The assembly described in this reference is not effective for polyethylene owing to its higher expansion rate and greater tendency to bend or for larger diameter pipes that present larger expansion forces. A different approach is thus required.